Through an investigation of glottalisation in several languages of North America, this article examines the role of perception in the grammatical distribution of segments. Specifically, we consider the hypothesis that perceptual factors on the timing of glottalisation in consonants determine the arrangement of such consonants phonologically. Our conclusion is in the negative. In the cases we study, the distribution of glottalisation appears to be governed by syllable structure, not by glottal timing. We interpret the patterns presented as evidence for a modular theory of phonetics and phonology rather than a theory where properties of articulator timing are directly referenced by phonological constraints. Patterns of glottalisation result from the interaction of phonological submodules that are largely independent of each other, though properties of particular modules may be influenced by phonetic factors.